Weakening Hurricane Irma has submerged streets, knocked out power to millions and snapped massive construction cranes over Miami's skyline as the storm continues to move north along Florida's west coast.

Key points:

At least one person has been killed in the US as tens of thousands take to shelter

The storm system has been downgraded to category one

Irma is expected to cause billions of dollars in damage to the third biggest US state

The eye of the storm, now downgraded to category one, is moving towards the highly populated Tampa Bay area, where residents are fearing the worst.

Tampa Bay is particularly vulnerable because the bay acts as a funnel for storm surges, forcing water into narrow channels with nowhere else to go.

All of southern Florida felt the storm's effects, with at least one man killed, a woman forced to deliver her own baby and trees and apartment towers swaying in high winds.

The storm, one of the most powerful ever recorded in the Atlantic, made landfall on Sunday as a category four storm with 210kph winds in the Florida Keys off the state's southern tip at 9:10am local time (11:10pm AEST) — hours later it was downgraded to a powerful category three storm.

Irma headed for the state's west coast with 195kph winds and storm surges — water driven ashore by the winds — of up to 4.6 metres were forecast, the National Hurricane Centre (NHC) said.

US President Donald Trump declared a major disaster in Florida, making federal aid available to people affected by Hurricane Irma in nine counties already hit by the storm.

"Right now, we are worried about lives, not cost," Mr Trump said.

US authorities say the storm, which left more than 20 people dead in the Caribbean, has already claimed at least one life in Florida — emergency responders in the Florida Keys said they pulled a man's body from his pickup truck, which had crashed into a tree in high winds.

More than 1.8 million Florida homes and businesses had lost power by noon (local time) as the storm pummelled the southern part of the state, utility Florida Power & Light (FPL) said, with repairs to its systems expected to take weeks.

"We will have to rebuild part of our system, particularly in the western part of the state. That restoration process will be measured in weeks, not days," FPL spokesman Rob Gould said.

A Miami woman who went into labour was guided through delivery by phone when authorities could not reach her in high winds and street flooding. Firefighters later took her to the hospital.

An apparent tornado spun off by Irma destroyed six mobile homes in Palm Bay, hundreds of miles away along the state's Atlantic coast. Flooding was reported along Interstate 4, which cuts across Florida's midsection

"There is a serious threat of significant storm surge flooding along the entire west coast of Florida," Mr Scott told a press conference.

"This is a life-threatening situation."

"I am very concerned about the west coast. This storm surge is just deadly."

Earlier, Mr Scott had warned residentsin the state's evacuation zones on Saturday that "this is your last chance to make a good decision". About 6.4 million people were told to flee.

But because the storm is 900 to 1,000 kilometres wide, the entire Florida peninsula was exposed. Forecasters said the greater Miami area of 6 million people could still get life-threatening hurricane winds and storm surge of 1.2 to 1.8 metres.

Meteorologists predicted Irma would plough into the Tampa Bay area on Monday morning.

The area has not been struck by a major hurricane since 1921, when its population was about 10,000, National Hurricane Centre spokesman Dennis Feltgen said. Now around 3 million people live there.

Irma's course change caught many off guard and triggered a major round of last-minute evacuations in the Tampa area.

Some locals grumbled about the forecast, even though Florida's west coast had long been included in the zone of probability.

"For five days, we were told it was going to be on the east coast, and then 24 hours before it hits, we're now told it's coming up the west coast," Jeff Beerbohm, a 52-year-old entrepreneur in St Petersburg, said.

Nearly the entire Florida coastline remained under hurricane watches and warnings, and the latest projections could shift again, sparing or savaging other parts of the state.

"This is going to sneak up on people," Jamie Rhome, the head of the hurricane centre's storm surge unit, said.